Coastal downwelling is usually thought to be biologically non-productive and biological carbon pump is not important and thus, its effects on carbon sequestration is often overlooked. In the continental shelf of the Northern South China Sea (NSCS), coastal downwelling occurs in winter. Our objectives in this study are 1) to examine whether there is coastal downwelling in the east coast off the Hainan Island and 2) to analyze its effects on carbon sequestration. We used vertical profiles of temperature, salinity, and DIC in March 2018 along the transects in the coastal region. The results show that strong downwelling occurred along the coastal area and was intensified by the steep continental shelf. Based on the T-S diagrams in the region, the three end members model was established to estimate relative contributions of three water masses to carbon concentrations. The results show that coastal surface water rich in dissolved inorganic carbon (DIC) were carried to the bottom by the downwelling, and further transported to below the permanent pycnocline offshore, becoming carbon sequestration in the deep ocean. The estimated DIC sequestration in the downwelling area was ~1.45TgC. The findings demonstrate that coastal downwelling is an important process for carbon sequestration in the east coast of Hainan Island, which has implications for other coastal areas in estimating carbon sink.

downwelling, carbon sequestration, Hainan Island